See City of Boston, 26 MLC 144, 147, MUP-1085, March 10, 2000; Town of Natick, 11 MLC 1434, 1438, MUP-5319, February 19, 1985. The CERB has previously determined that an employer may not unilaterally alter a pre-existing pattern of shared work. See City of Boston, 28 MLC 194, 195, MUP-2185, January 2, 2002; City of Quincy, Quincy City Hospital, 15 MLC 1239, 1241, MUP-6490, November 9, 1988; City of Boston, 6 MLC 1117, MUP-2863, June 4, 1979 .